Picture yourself in the role of evolution. You are presented with a particular phenotype, you look at it, look at the environment in which it lives and decide how fit it's going to be.

This is a terrible way of thinking about it. Evolution doesn't think, doesn't plan, doesn't care how many species go extinct. It's not striving toward some specific goal. It just tinkers and kills. Your burden, if you are going to claim that this is insufficient, is to come up with a specific case where it is demonstrably insufficient. Not a contrived example with no connection to the real world.

To belabor the point:

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Now I agree with you that this would work if we assumed that the pace of change in the environment would be slow, and maybe evolution had some reason for making that assumption (apologies again for the anthropomorphization).

Don't apologize, just don't do it. Evolution doesn't make assumptions. We observe that the pace of change is slow enough that life has been able to adapt to it so far. Based on this observation, evolution appears to be a sufficient explanation. If you want to claim this is not so, come up with some specifics.

1) Vision (like most traits of living creatures) is sloppy. A good fraction of humans are dichromats, a few are monochromats, and some may even be tetrachromats. Trichromats appear to be favored, but one can easily imagine a slightly different history where we ended up dichromat, like many other successful species. Evolution didn't pick some single right answer in advance, it just rolled the dice and we adapted to the result. If the dice landed differently, we would have adapted differently, or gone extinct.

2) The hypothetical case of the world suddenly strongly favoring grue/bleen is fantasy. Evolution doesn't account for it, which is fine, because it hasn't happened.

3) If it did happen, the sloppiness in 1) suggests that it wouldn't necessarily be fatal. If were fatal... so what ? The vast majority of species that have existed are extinct, precisely because they faced contingencies that evolution didn't "plan" for. The same could happen to us at any moment. We are, in essence, the descendants of a long line of lottery winners. That sounds unlikely (or maybe a sign of outside interference, as the ID camp would like to claim), until you realize that everyone with a losing ticket has been shot, and the winners have had enough children to replace them.